In a magnetic resonance imaging (MRI) system, specific absorption rate (SAR) is a safety indicator. Since specific absorption rate is in proportion to the square of the field intensity, specific absorption rate is a critical problem in a magnetic resonance imaging system with a high field intensity. A body coil is a critical component for transferring energy. Relatively high local specific absorption rate means that the average radio frequency power applied to the body coil is to be limited, but the limiting will increase the scanning time. Hence, it is very important to reduce the local specific absorption rate with respect to the design of body coils for magnetic resonance imaging systems with high field intensities.
FIG. 1 is a schematic diagram of a body coil with an end ring structure for magnetic resonance imaging equipment in the prior art. As shown in FIG. 1, reference numeral 101 indicates a cylinder for housing a patient in magnetic resonance imaging equipment. 102 indicates a bird-cage body coil, and there is an end ring 103 and 104 respectively at each of the two ends of the bird-cage body coil. 105 indicates a leg (e.g., metal strip) for connecting two end rings, and 106 indicates that there is a capacitor at a position on each end ring between two metal strips.
In a high-pass bird-cage body coil structure, the worst situation is where the local specific absorption rate of radio frequency is located near the end ring. The design of the end ring is a critical factor to reduce local specific absorption rate.
The reasons for relatively high local specific absorption rate may be divided into two aspects. One is electric field coupling, and the other is magnetic field coupling. Usually, a method for solving the problem of relatively high local specific absorption rate is to reduce the electric field coupling effect. A first method is to replace each capacitor on the end ring with two or more capacitors connected in series. Such a method may be used to lower the voltage across each capacitor that will reduce the electric field coupling effect. A second method is to add foam underneath the end ring that will change the dielectric constant of the material between the end ring and the patient. A third method is to increase the distance between the end ring and the patient. All the above-mentioned methods are on the basis of reducing electric field coupling, but these methods have problems such as high costs, loss of internal space of the cylinder, and so on. Moreover, if the local specific absorption rate is dominated by the magnetic field, the above-mentioned methods may not have the desired effects.